Katy Perry's Revenge 2, free

I think the title of this app alone is worth downloading it. If you've ever played Tap Tap you know how addicting it can be- you simply tap circles to the beat of a song, similar to DDR or Guitar Hero. Katy Perry's Revenge is two free Tap Tap games to her latest songs Firework and E.T. The full versions of the songs play as beats zoom toward you and fireworks shoot around in the background. I'm embarrassed by how fun this is.